In this work, we measured the photovoltaic responses of two structurally similar small-molecule donors synthesized at Heliatek GmbH—DCV-Me and DCV-iPr—which exhibit strikingly different performances despite differing by only one isopropyl substituent. To uncover the origin of this contrast, we performed a comprehensive experimental–theoretical study, combining crystal structure prediction, vapor deposition simulations, single-crystal X-ray diffraction, GIWAXS analysis, photovoltaic characterization, voltage loss studies, as well as state-of-the-art exciton-vibronic calculations to simulate the optical steady-state spectra and relate the differences to the distinct packing motifs of the two systems.

We find that steric effects from the isopropyl group in DCV-iPr result in a brick-wall molecular arrangement, resolved in the crystal structure and preserved in evaporated thin films. Our excitonic calculations reveal that this geometry promotes J-like excitonic interactions, sharp absorption features, and a reduced Stokes shift—favorable for efficient solar energy conversion. In contrast, as predicted by our theoretical model and confirmed experimentally, DCV-Me forms H-like aggregates with broadened absorption, increased voltage losses and poor photoconversion efficiency. 

We next investigate the aggregation behavior of dipyrrolonaphthyridinedione (DPND) chromophores in the solid state critically determines their optoelectronic properties. Here, we investigate how systematic variation in the side-chain geometry—specifically the branching point and steric profile—governs molecular packing and excitonic coupling. Using crystal structure prediction (CSP) combined with experimental GIWAXS and solid-state NMR, we obtain the packing geometry and crystal structure for three DPND derivatives (DPND-iPr, DPND-EtPr₂, and DPND-iBu). The results reveal that side-chain branching at the first carbon atom promotes herringbone packing and J-type behavior, while branching at the second carbon induces brick-wall stacking and H-type behavior in the solid state. Optical simulations based on the Holstein exciton-vibrational Hamiltonian reproduce experimental absorption and photoluminescence spectra, confirming the transition from J-like to H-like photophysics as the side-chain branching position shifts. This study demonstrates that fine-tuning alkyl side-chain geometry enables rational control of aggregation and excitonic behavior in cross-conjugated DPNDs, providing new design principles for functional organic semiconductors.